1. Field of the Invention
The present invention generally relates to the manufacture of chemical vapor deposition reactors and, more particularly, to reflective surfaces within such reactors.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a very well known process in the semiconductor industry for forming thin films of materials on wafers. In a CVD process, gaseous molecules of the material to be deposited are supplied to wafers to form a thin film of that material on wafers by chemical reaction. Such formed thin films may be polycrystalline, amorphous or epitaxial. Typically, CVD processes are conducted at elevated temperatures, to accelerate the chemical reaction and produce high quality films.
In the semiconductor industry, it is important that the material be deposited uniformly thick with uniform properties over the wafer. In Very Large and Ultra Large Scale Integrated Circuit (VLSI and ULSI) technologies, the wafer is divided into individual chips having integrated circuits thereon. If a CVD process step produces deposited layers with nonuniformities, devices at different areas on the chips may have inconsistent operation characteristics, or may fail altogether.
One of the most important factors in achieving uniformly thick and high quality thin films is the uniformity of temperature within the chamber, and particularly of the temperature across the semiconductor wafers (or other deposition substrates). Substrates can be heated using resistance heating, induction heating or radiant heating. Among these, radiant heating is the most efficient technique and hence is currently the favored method of supplying energy to a CVD chamber.
Significantly, radiant heating results in short processing times and greater throughput. Radiant heating directly heats the wafer during the CVD process. The temperature of the wafers can be ramped up to the desired process temperature, and ramped down to a satisfactory handling temperature, faster than with alternative heating techniques. Additionally, radiant heating can be controlled to maintain the wafer at the desired temperature for a sufficient time to accomplish the process step. Radiant heating energy can be supplied, for example, by banks of quartz halogen lamps above and below the reaction chamber.
Unfortunately, radiant energy has a tendency to create nonuniform temperature distributions, including "hot spots," due to the use of localized sources and the consequent focusing and interference effects.
In an effort to provide more uniform illumination and resulting uniform temperature distribution across the wafers, the industry practice has been to mount reflectors behind the lamps to indirectly illuminate the wafers. These reflectors are generally made of a base metal and often are gold plated to increase their reflectivity. Planar reflecting surfaces, however, still tend to induce hot spots on wafers being heated.
Accordingly, a need exists for a system for achieving uniform temperature distributions across semiconductor wafers during processing. Desirably, such a system should maintain the advantages of radiant heating.